1. Field of the Invention
This invention relates to an image forming method and a heat-development type photosensitive material to be used therefor. More particularly, the invention is concerned with the image forming method which uses the heat-development type photosensitive material containing therein an organic silver salt, and such heat-development type photosensitive material suited for the purpose.
2. Description of the Prior Arts
Photosensitive materials containing therein silver halide have heretofore been used widely in forming a picture image. As these photosensitive materials, however, are of wet type in their development and image fixation, there are various disadvantages still to be improved such that the treatment process is complicated and takes much time, that the treatment process must be conducted in a dark room, that undesirable effects are likely to be caused to human body working in such chemical treatment in the dark room, and others.
Various attempts have been made in recent years to carry out the treatment in a dry condition rather than in a wet condition. The most representative of such dry type treatment is as taught in U.S. Pat. No. 3,457,075, wherein a heat-development type photosensitive material containing therein a catalytic quantity of a photosensitive silver halide principally composed of an organic silver salt and a reducing agent is subjected to a heat-treatment after an image exposure thereon to obtain a visible picture image.
This image forming method is of such type that, when the abovementioned heat-development type photosensitive material is subjected to an image exposure, a very small amount of silver is isolated from the photosensitive silver halide in the catalytic quantity to form a latent image, and this small amount of isolated silver functions, during the subsequent heat-development process, as a nucleus for development of silver which is isolated from the organic silver salt and deposited with the aid of the reducing agent, thereby forming a visible picture image.
Accordingly, the image forming process is totally of the dry type, which process has drawn increasing concern from all fields. And, the improvement and development in the heat-development type photosensitive material used therefor are very active. At the same time, this image forming method has been attaining wider applications in various aspects such as use as an electrostatic printing master which is disclosed in U.S. Pat. No. 599,061 filed by the present applicants, and as recording material in many fields.
While the heat-development type photosensitive material to be used for such image forming method has its advantage in that it is capable of carrying out the entire treatment process in the dry condition, it has various disadvantageous problems to occur at the time of the actual image formation owing to the entire substances required for the image formation being made coexistent from the initial stage of preparing the heat-development type photosensitive material. That is, there is a considerable lapse of time from preparation of the heat-development type photosensitive material to the actual image forming operation using the same. In other words, there is a considerable lapse of time ranging from an order of several weeks to even more prolonged time lapse after the photosensitive material is prepared at the factory until it is used for the image formation at the users who purchased the material through a distributor, or after the users purchased the heat-devlopment type photosensitive material until they actually use the material for the image reproduction. It becomes, therefore, very important that the same degree of performance of the photosensitive material as at the initial stage of its preparation be maintained throughout this prolonged time period. For this purpose, there have been taken various measures such as, for example, tight sealing of the heat-development type photosensitive material in, for example, a paper envelope lined its inside with an aluminum laminate, and so forth. As a matter of practice, however, when such tightly sealed envelope is opened at its one end for use of the photosensitive material, there inevitably remain a number of unused sheets, and these unused sheets are liable to be exposed to the natural atmosphere with the results that they no longer maintain their initial performance and becomes deteriorated. Such situation takes place not infrequently. To avoid such undesirable consequence, it is necessary that, when the heat-development type photosensitive material is made into a sheet form, each sheet to be completely sealed in an individual envelope as mentioned above, or that, in the case of such sheets being enveloped in bulk and the envelope being once opened for use of some of the sheets, the envelope be tightly sealed again immediately after the required number of sheets are taken out of the envelope, or that the sheets remaining unused in the open envelope be kept in a desired atmosphere to maintain their performance, and so forth. Any of these measures, however, results in waste of time and labor, which is undesirable from the viewpoint of saving expensive personnel cost and resources, and of required promptness in dissemination of informations, and so forth.
Further, when the heat-development type photosensitive material is used as a forming material of the electrostatic printing master as described in the specification of U.S. patent application Ser. No. 599,061 filed by the present applicants, the abovementioned points constitute extreme inconveniences. That is to say, the abovementioned electrostatic printing master is obtained by subjecting the heat-development type photosensitive material to an image exposure and heat-development treatments to form a silver image pattern therein, the electrostatic printing method of which is to subject the electrostatic printing master having such silver image pattern to electrostatic charging, development of an image, and transfer of the developed image. The principle of the electrostatic printing is based on the fact that, at the time of the electrostatic charging step, the exposed portion (silver image portion) of the master is relatively electrically conductive and relatively small in its electrostatic charge sustaining capability, while the non-exposed portion (non-silver image portion) has such electrostatic charge sustaining capability, owing to which there occurs an electrostatic potential contrast. Therefore, deterioration in the image forming capability with lapse of time of the heat-development type photosensitive material as the forming material of the electrostatic printing master inevitably invites, for example, insufficiency of the silver deposition at the exposed portion of the photosensitive material to unfavorably cause the quality of the transferred image to lower due to decrease in the electrostatic potential contrast.
For the abovementioned reasons, there has been called for development of a heat-development type photosensitive material excellent in its preserving property, i.e., excellent in maintaining its image forming capability comparable to the quality at its initial stage of preparation, or new proposal for the image forming method. Furthermore, from the standpoint of the material for forming the electrostatic printing master proposed by the present inventors in their previous application, the development of the heat-development type photosensitive material, with which the electrostatic printing master having such excellent electrostatic characteristic can be formed, is all the more strongly desired.
In view of the foregoing, the present inventors conducted diligent researches and studies from various aspects on the heat-development type photosensitive material containing therein an organic silver salt, as the result of which they have found that inclusion of a development accelerator represented by a reducing agent from the initial stage of preparing the photosensitive material constitutes the very point of problem in the deterioration of the image forming quality of the material.
While succinct theory for the cause of the deterioration in the image forming capability of the heat-development type photosensitive material due to inclusion of such development accelerator from the initial stage of preparing the photosensitive material has not yet been established, it may be assumed as follows. That is, in the known type of heat-development type photosensitive material, as the development accelerator representable by a reducing agent and which is one of the components of the photosensitive material is included in the material from the initial stage of its preparation, the condition of existence of these components in the heat-development type photosensitive material changes from that at its initial stage of preparation in the course of its storage over a long period of time. In other words, the state of coexistence of the organic silver salt and the development accelerator to impart a preferable performance of the photosensitive material at the initial stage of its preparation appears to change with lapse of time to cause the aforementioned problems to occur, or the development accelerator per se deteriorates with lapse of time due to, for example, its scattering outside of the system through evaporation, or both decomposition and evaporation.